This delimits all batch operations which access memory between
iris_batch_sync_region_start() and iris_batch_sync_region_end() calls.
This makes sure that any buffer objects accessed within the region are
considered in use through the same caching domain until the end of the
region.

Adding any buffer to the batch validation list outside of a sync
region will lead to an assertion failure in a future commit, unless
the caller explicitly opted out of the cache tracking mechanism.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Part-of: <mesa!3875>